In one class of known PLDs, each device has a large number of logic gates, and a user programs the device to assume a particular configuration of those logic gates, frequently using a software tool provided by the manufacturer of the device, with the software tool being executed on a computer having an adapter into which the device is inserted. Early generations of such devices typically used some form of programmable read only memory (“PROM”) technology to store the configuration data produced by the software tool. In those early devices, the software tool caused the computer to “burn” the pattern into the PROM storage by fusing fusible links. Later, programmable logic devices that store their configuration data in static random access memory (“SRAM”) storage became available and remain prevalent. However, SRAM storage is volatile; it does not retain its contents when power is lost. Therefore, programmable logic devices based on SRAM technology are used with nonvolatile storage as well, to retain the configuration programming data during times that the device is switched off or otherwise not provided with power.
Many applications employing PLDs have critical security requirements. For example, governments and corporations invest heavily in critical networking infrastructures, sophisticated weapon systems, and secure banking systems. Additionally, the circuit and algorithm designs that are implemented in PLDs represent important intellectual property for developers, and thus need to be secured against competitors who seek to copy or interfere with these designs. A number of security features may be used to mitigate against reverse engineering, tampering, and other security risks, such as various levels of encryption. These features are typically fixed in the silicon of the programmable logic device at the time of manufacture. As a result, PLD manufacturers must produce a different device for each security level, and customers must know at the time of purchase exactly which security features are required or desirable for their application. Additionally, devices with different security levels are subject to different export regulations, complicating sale and distribution of these devices.